Ceramic materials containing certain mixed metal oxide compositions possess both oxygen ion conductivity and electronic conductivity at elevated temperatures. These materials, known in the art as mixed conducting metal oxides, may be used in applications including gas separation membranes and membrane oxidation reactors. Ceramic membranes are made of selected mixed metal oxide compositions and have been described as ion transport membranes (ITM). Catalysts may be used, for example, to promote the oxidation and/or reforming reactions that take place in membrane oxidation reactors.
A characteristic property of these materials is that their oxygen stoichiometry is a thermodynamic function of temperature and oxygen partial pressure wherein the equilibrium oxygen stoichiometry decreases with increasing temperature and with decreasing oxygen partial pressure. It is known that the dimensions of all materials change with changing temperature due to thermal expansion and contraction. In addition to these thermal dimensional changes, mixed conducting metal oxide materials undergo chemical dimensional changes that are functions of the metal oxide oxygen stoichiometry. At isothermal conditions, an article made of mixed conducting metal oxide material will increase in dimensions with decreasing oxygen stoichiometry. At isothermal conditions, the oxygen stoichiometry decreases with decreasing oxygen partial pressure. Since the equilibrium oxygen stoichiometry increases with decreasing temperature, an article made of mixed conducting metal oxides will contract due to both thermal and chemical dimensional changes as the temperature decreases at a constant oxygen partial pressure. Conversely, an article made of mixed conducting metal oxides will expand by both thermal and chemical dimensional changes as the temperature increases at a constant oxygen partial pressure. This is described in an article entitled “Chemical Expansivity of Electrochemical Ceramics” by S. B. Adler in J. Am. Ceram. Soc. 84 (9) 2117-19 (2001).
Dimensional changes therefore result from equilibrium oxygen stoichiometry changes in mixed conducting metal oxide materials. Changing the temperature at a constant oxygen partial pressure or changing the oxygen partial pressure at a constant temperature will change the equilibrium oxygen stoichiometry of the mixed conducting metal oxide material. When a mixed conducting metal oxide is used as an ion transport membrane, for example, an oxygen partial pressure difference across the membrane creates a difference in the equilibrium oxygen stoichiometry at each of the two surfaces of the membrane, which in turn creates the thermodynamic driving force for oxygen ions to diffuse through the membrane.
It is known that temperature gradients in a mixed conducting metal oxide ceramic structure can create differential strains due to differential thermal expansion and contraction. Similarly, oxygen stoichiometry gradients in a ceramic structure can create differential strains due to differential chemical expansion and contraction. This gradient in oxygen stoichiometry may be sufficiently large to create a correspondingly large differential chemical expansion, and therefore large mechanical stresses, that lead to failure of the ceramic structure. It is desirable, therefore, to avoid differential chemical expansion or at least to control the differential chemical expansion to below maximum allowable values.
There is a need in the art for improved methods to reduce the potential for mechanical damage due to dimensional changes during the heating and cooling of mixed conducting metal oxide membrane systems, particularly in the operation of membrane reactor systems under transient conditions of temperature, pressure, and gas composition that may occur during membrane and module fabrication as well as subsequent startup and shutdown of the membrane systems. These needs are addressed by embodiments of the invention disclosed below and defined by the claims that follow.